Liquid biofuel made of esters and bound glycerides, and also process for production thereof

ABSTRACT

The present invention relates to a biofuel and to a process for production thereof, which biofuel contains at least one fraction of triglycerides, a fraction of monoglycerides, a fraction of diglycerides and a fraction of alkylesters of fatty acids. The biofuel may be obtained by partial transesterification of vegetable oil or vegetable fat and is distinguished in that the mass fraction of alkylesters of fatty acids in the biofuel is smaller than the mass fraction of triglycerides. The fuel is suitable in particular as additive for conventional fuels, may be produced at high yield and can also be used at relatively low temperatures below 10° C.

TECHNICAL FIELD

The present invention relates to a liquid biofuel having a basis of triglycerides, mono- and diglycerides, and alkylesters of fatty acids, and a process for production thereof. The fuel is particularly suitable as a substitute for conventional fuels, for example diesel. It is also possible to use it directly as a fuel in combustion engines.

STATE OF THE ART

Biofuels and biofuel mixtures based on vegetable oil and animal fat are described for example in Patent Nos. DE 4116905 C1, WO 95/25152 A1, EP 855436 A2, or U.S. Pat. No. 5,713,965A. These documents particularly disclose mixtures of rapeseed oils with petrol or diesel, to which an additional substance is added. In German Patent No. DE 4116905 C1, this additional component is an alcohol, in WO 95/25152 A1, it is an alkylester of a short-chain fatty acid having a maximum chain length of 6 C atoms, and in EP 855436 A2 it is an acetal.

Patent specifications U.S. Pat. No. 5,713,965 A, U.S. Pat. No. 5,480,787 A and EP 1705238 A1 (=WO 2005075615 A1) describe processes for the production of alkylesters of fatty acids by lipase-catalysed transesterification of oils and fats. In these processes, the fatty acid alkylesters of bound and free glycerol are separated by known methods and accordingly are not used as a fuel fraction. Since the glycerol is separated, the yield relative to the feedstock oil or fat is only about 90%.

Patent specification EP 1126011 A2 describes a process for homoegenously catalysed transesterification of fats and oils under conditions in which at least one of the reactants is present in a critical state. As with the specifications cited in the preceding, U.S. Pat. No. 5,713,965 A, U.S. Pat. No. 5,480,787 A, and EP 1705238 A1, in this case too the glycerol produced is separated from the fatty acid alkylesters and is not used as a fuel fraction.

Patent specification U.S. Pat. No. 55,788,090 A describes a biofuel consisting of fatty acid alkylesters and bound glycerol. Unlike the fuel described in the present document, the bound glycerides are not present in the form of mono-, di-, and trialkylglycerol esters, but as mono-, di-, and trialkylglycerol ethers. These ethers are produced in a reaction that differs from transesterification, as intermediate products of the transesterification reaction.

Patent specification U.S. Pat. No. 5,316,927 A describes a process for producing monoglycerides and fatty acid alkylesters via lipase-catalysed transesterification of fats and oils. When the reaction is complete, the two products are separated from one another and fed to different applications.

Diesel fuel is a refined petroleum product that represents a considerable portion of the international fuel market. Finite resources, dramatic price increases, and the ongoing debate regarding climate have led to renewed efforts to at least partially replace fossil diesel with fuels from renewable raw materials.

At the moment, the most of such raw materials are drawn from vegetable oil and biodiesel (alkylesters of fatty acids). Vegetable oils largely consist of triglycerides, that is to say esters of glycerol, and three fatty acids, and in smaller quantities, free fatty acids.

The use of pure vegetable oil as diesel fuel is not without certain difficulties. Because of its high viscosity and low cetane number, the triglycerides are only partially combusted, which results in deposits on valves and fuel injectors, and high emission values. Mixing vegetable oil with fossil diesel also presents problems, because such mixtures are unstable, particularly at low temperatures, and consequently there is a danger that they will separate in the fuel tank.

These problems can be largely avoided by transesterification of triglycerides with a monovalent alcohol. The fatty acid alkylesters produced have a viscosity and cetane number similar to those of diesel, and are thus able to be used as a diesel substitute relatively easily.

Besides alkylesters of fatty acids (biodiesel), transesterifying vegetable or animal oils or fats also produces about 10% free glycerol. Free glycerol is insoluble in biodiesel and therefore cannot be used as a fuel fraction. The separated free glycerol lowers the fuel yield of the process, and since it needs to undergo downstream processing and the revenue situation therefrom is unfavourable, it represents a not insignificant cost factor.

A fuel that is produced by partial transesterification of triglycerides is described in PCT/DE2005/002156. In this case, the fuel yield is 100%, because the glycerol contained in the fats and oils is not liberated, but is kept in solution in the form of mono-, di-, and triglycerides. This mixture of bound glycerol and fatty acid alkylesters is stable at room temperature. At lower temperatures however, below 10° C., crystallisation processes are triggered and individual components are precipitated, particularly with the compositions described as especially advantageous in PCT/DE2005/002156. Consequently, the fuel described in PCT/DE2005/002156 is only suitable for use at room temperature or higher temperatures. The fuel described is therefore not suitable precisely in regions where low temperatures prevail.

The object of the present invention consists in providing a biofuel, and a process for production thereof, which may be produced with a high yield and is also usable at lower temperatures.

DESCRIPTION OF THE INVENTION

The object is solved with the biofuel according to claim 1 and the process according to claim 13. Advantageous compositions of the biofuel and configurations of the processes for production thereof are described in the subordinate claims or will be evident from the following description and embodiments.

The biofuel according to the invention contains at least one fraction of triglycerides, particularly vegetable oil or vegetable fat, and at least one fraction of monoglycerides and at least one fraction of diglycerides, and further contains at least one fraction of alkylesters of fatty acids.

Surprisingly, it was found that there is a defined range of compositions with the fractions cited, for which, despite a lower viscosity and higher cetane number, it is possible, by partial transesterification, to keep all of the glycerol contained in the oil in solution in the form of mono-, di- and triglycerides, and to store the product even at temperatures below 10° C., without demixing or crystallisation. Such demixing does not occur even after the biofuel according to the invention is mixed with diesel fuel, although mixing with diesel does cause the polarity of the mixture to change. Mixtures of the biofuel according to the invention with diesel thus remain clear and monophasic even in wintry conditions.

It was found that in order to achieve optimum solubility of glycerides in fatty acid alkylesters (FAAEs) even at lower temperatures, it is critically important that the fraction of FAAEs in the fuel is smaller than the fraction of triglycerides. The triglycerides dissolve particularly readily even at temperatures below 10° C. if the mass fraction of the triglyceride content is selected to be greater than 29%, advantageously greater than 40%, and the mass fraction of the FAAE is adjusted to greater than 14% and less than 36%.

A further factor for increasing the solubility of glycerides in FAAE-containing fuels is the ratio of monoglycerides to diglycerides.

If the ratio of diglycerides to monoglycerides is below a value of 2, particles are precipitated particularly rapidly.

The following composition contains a mixture of mono-, di-, and triglycerides with FAAE that is particularly advantageous for cold storage:

Minimum mass Maximum mass Component % value % value FAAE 15 35 Monoglycerides 0.1 10 Diglycerides 4 30 Triglycerides 30 65

A mixture having a composition of 20-25% by mass FAAE, 50-55% by mass triglycerides, 20% by mass diglycerides, and 5% by mass monoglycerides proved to be particularly stable at low temperatures. A further improvement in terms of low-temperature stability may be achieved by adding up to 2% by mass of ethanol.

The biofuel may be mixed in any ratio with fossil fuel, biodiesel or BTL-fuel, and in this context it may be diluted and used as fuel for internal combustion engines. It is also possible to dilute the fuel according to the invention by adding diesel fuel or biodiesel before the partial esterification of the triglycerides.

It is also expedient to use mono- and diglycerides originating from another reaction in the biofuel, such as those that are formed when vegetable oil is transesterified into fatty acid alkylesters. However, it is also possible, and under certain circumstances may be advantageous, to use mono-, di-, and triglycerides that originate from another source, if applicable from an animal source, or are possibly of synthetic origin. For example, it is possible to use mono- and diglycerides in the biofuel that contain fatty acids with fewer than 10 carbon atoms.

One possible production process for the suggested biofuel is based on partial esterification of triglycerides. For this, triglycerides are mixed with an alcohol and a reaction is provoked by adding a catalyst or placing them in contact with a catalyst. The triglycerides used may be a raw material of vegetable, animal or synthetic origin, or mixtures of triglycerides from different sources.

The alcohol used is preferably a monovalent alcohol with any chain length. Either organic or inorganic compounds, or enzymes or microorganisms may be used as the catalyst. The reaction may be triggered by homogeneous or heterogeneous catalysis. In a financially particularly advantageous form, used cooking fats serve as the raw material source.

The composition of the biofuel, particularly the ratio of the mass fractions of fatty acid alkylesters and triglycerides and/or the mass fractions of diglycerides relative to the monoglycerides, may be adjusted via the residence time, the type and quantity of catalytic material, and the quantity of alcohol used. In this context, residence time is considered to be the period for which the catalyst and the added alcohol are in contact with the triglycerides.

Use of carrier-bound sn-1,3 regiospecific lipases as the catalyst is particularly advantageous. In this context, adding enzymes incrementally over the course of several discontinuous production cycles has proven to produce a particularly good yield. In each production cycle, the enzyme from the previous cycle is used, a small additional quantity of fresh enzyme being added in each cycle to obtain a particularly good yield.

It was also discovered that the gradual addition of the alcohol in sub-stoichiometric quantities in several steps or continuous addition of alcohol is particularly advantageous. The gradual or continuous addition of alcohol is to be set up as far as possible so that the concentration of alcohol does not exceed 4% by mass. A maximum alcohol concentration of 3% by mass has proven particularly advantageous for high stability and thus also a long service life of the enzyme.

It has also proven advantageous to the same purpose if the alcohol is dissolved in a reaction mixture that is largely free from lipases, and is not brought into contact with the lipases again until the alcohol is completely dissolved. In this context, lipase-free product is removed from the reaction vessel several times during the production cycle for use in dissolving the alcohol that is needed for the reaction. In this way, it is possible to avoid bringing high alcohol concentrations into contact with the enzyme. In this context it was found that the alcohol concentration in the reaction mixture described should not exceed 5% before it is remixed with the lipase.

The process temperature is determined by the catalyst employed, and on the triglyceride used, in particular by the melting point thereof. The residence time is determined by the catalyst employed, the quantity of the catalyst, the alcohol used, and the triglyceride used.

Embodiment

2.0 g ethanol are fully dissolved in 100 g rapeseed oil. The transesterification reaction is started by adding 1.0 g of an immobilised s_(n)-1,3 regiospecific lipase. The mixture is mixed thoroughly for 3 hours at the temperature of highest lipase activity.

After 3 hours, 50 mL of lipase-free intermediate product is removed from the reaction vessel. 2.0 g of ethanol are completely dissolved in the reaction medium that has been removed. Then, the product-ethanol solution is returned to the reaction vessel. This step is then repeated after a further 3 hours' residence time.

After a total of 10 hours, the lipase is separated from the reaction product, and a clear, monophasic liquid is obtained, consisting of 30% by mass fatty acid ethyl ester, 24% by mass diglycerides, 9% by mass monoglycerides, and 36% by mass triglycerides, and about 1% by mass ethanol.

The separated enzyme may be reused in a second production cycle. This is performed in identical manner to the cycle described in the preceding. At the start of the reaction, 0.1 g fresh enzyme is added to the enzyme that was used in the first production cycle. 

1. Biofuel that contains at least one fraction of triglycerides, a fraction of monoglycerides, a fraction of diglycerides and a fraction of alkylesters of fatty acids, characterized in that the mass fraction of alkylesters of fatty acids in the biofuel is smaller than the mass fraction of triglycerides.
 2. The biofuel according to claim 1, characterized in that the ratio of mass fractions of diglycerides to monoglycerides is ≧2.
 3. The biofuel according to either of claim 1 or 2, characterized in that the mass fraction of alkylesters of fatty acids is not less than 15% and not more than 35%.
 4. The biofuel according to either of claim 1 or 2, characterized in that the mass fraction of triglycerides is at least 30%.
 5. The biofuel according to claim 4, characterized in that the mass fraction of triglycerides is greater than 40%.
 6. The biofuel according to claim 5, characterized in that the mass fraction of triglycerides is not more than 65%.
 7. The biofuel according to either of claim 1 or 2, characterized in that the mass fraction of monoglycerides is between 0.1% and 10%.
 8. The biofuel according to either of claim 1 or 2, characterized in that the mass fraction of diglycerides is between 4% and 30%.
 9. The biofuel according to claim 1, characterized in that the mass fraction of alkylesters of fatty acids is between 20% and 25%, the mass fraction of triglycerides is between 50% and 55%, the mass fraction of diglycerides is 20%, and the mass fraction of monoglycerides is 5%.
 10. The biofuel according to claim 1, characterized in that the biofuel contains an additive of ethanol having a mass fraction of up to 2%.
 11. The biofuel according to claim 1, which is mixed with fossil fuel, biodiesel or BTL-fuel.
 12. Use of the biofuel according to claim 1 as a fuel for internal combustion engines.
 13. Process for producing the biofuel according to any of claims 1 to 11 by partial transesterification of triglycerides, wherein at least one fraction each is obtained consisting of triglycerides, monoglycerides, diglycerides, and alkylesters of fatty acids, characterized in that the partial transesterification is carried out in such manner that the mass fraction of alkylesters of fatty acids in the biofuel is smaller than the mass fraction of triglycerides.
 14. The process according to claim 13, characterized in that the partial transesterification is carried out in such manner that the ratio of mass fractions of diglycerides to monoglycerides is ≧2.
 15. The process according to either of claim 13 or 14, characterized in that carrier-bound sn-1,3 regiospecific lipases are used as the catalyst for partial transesterification.
 16. The process according to claim 15, characterized in that the partial transesterification is completed with gradual addition of enzymes over the course of several discontinuous production cycles.
 17. The process according to either of claim 13 or 14, characterized in that alcohol is added in sub-stoichiometric quantity for the transesterification.
 18. The process according to claim 17, characterized in that alcohol is added continuously or in several steps during the transesterification.
 19. The process according to claim 18, characterized in that when alcohol is added in several steps, an alcohol concentration of 3% by mass is not exceeded.
 20. The process according to either of claim 13 or 14, characterized in that for transesterification, alcohol is first dissolved in a reaction mixture that is largely free from lipases and contains the triglycerides, and that the reaction mixture is not brought into contact with the lipases as the catalyst until the alcohol has been completely dissolved.
 21. The process according to claim 20, characterized in that an alcohol concentration of ≦5% by mass is adjusted in the reaction mixture before it is brought into contact with the lipases.
 22. The process according to either of claim 13 or 14, characterized in that the partial transesterification takes place with vegetable oil or vegetable fat as the triglycerides. 